Unruh deWitt detectors are important constructs in studying the dynamics of
    quantum fields in any geometric background. Curvature also plays an important
    role in setting up the correlations of a quantum field in a given spacetime.
    For instance, massless fields are known to have large correlations in de Sitter
    space as well as in certain class of Friedmann-Robertson-Walker (FRW)
    universes. However, some of the correlations are secular in nature while some
    are dynamic and spacetime dependent. An Unruh deWitt detector responds to such
    divergences differently in different spacetimes. In this work, we study the
    response rate of Unruh deWitt detectors which interact with quantum fields in
    FRW spacetimes. We consider both conventionally as well as derivatively coupled
    Unruh deWitt detectors. Particularly, we consider their interaction with
    massless scalar fields in FRW spacetimes and nearly massless scalar fields in
    de Sitter spacetime. We discuss how the term which gives rise to the infrared
    divergence in the massless limit in de Sitter spacetime manifests itself at the
    level of the response rate of these Unruh deWitt detectors in a wide class of
    Friedmann spacetimes. To carry out this study, we use an equivalence between
    massless scalar fields in FRW spacetimes with massive scalar fields in de
    Sitter spacetime. Further, we show that while the derivative coupling regulates
    the divergence appearing in de Sitter spacetime, it does not completely remove
    them in matter dominated universe. This gives rise to large transitions in the
    detector which can be used as a probe of setting up of large correlations in
    late time era of the universe as well. We show that the coupling of hydrogen
    atoms with gravitational waves takes a form that is similar to derivatively
    coupled UdW detectors and hence has significant observational implications as a
    probe of late time revival of quantum correlators.

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